1. Field of the Invention
This invention relates to methods and apparatus for the production of high density plasmas.
2. Description of Related Art
A variety of different processes are operated in a plasma environment including thin film coating, for example sputtering, evaporation, chemical vapour disposition, cleaning and etching processes.
For example, it has been known for many years that thin film deposition processes and the properties of the deposited films themselves can be Improved by allowing energetic ions to impinge on the growing surface of the film being deposited; this is due to an energy transfer between the energetic ions and the so-called xe2x80x9cadatomsxe2x80x9d of the thin film being deposited. This increases the surface mobility of the adatoms and allows them to migrate more readily to the preferred lattice sites.
Sputtering processes are widely used for the deposition of thin coatings or films of materials on to substrates. Such processes take place in an evacuated chamber containing a small quantity of an ionisable gas, for example argon. Electrons emitted from a source held within the chamber ionise the gas to form a plasma; a (cathode) target comprising the material to be sputtered is bombarded by the ions causing atoms of the target material to be dislodged and subsequently deposited on to the substrate being coated.
It is also well known that the rate of deposition in sputtering process may be increased by the use of magnetic means, for example an array of permanent magnets positioned in a predetermined manner (commonly as a closed loop) associated with the cathode target to create in use a plasma which is localised and concentrated along a sputtering zone of the target and thereby defined the area or region from which sputtering, or erosion of the target occurs.
In vapour deposition processes, especially plasma enhanced chemical vapour deposition (PECVD), a chemical gad contained within a vacuum chamber is dissociated and activated by the plasma at a rate generally proportional to the density of the plasma.
In plasma etching or cleaning processes, energetic ions and/or chemically active ions and/or chemically active ions produced by a plasma present in a vacuum chamber are employed to remove material from a substrate, for example in the production of semi conductor integrated circuits.
In all such processes, an important parameter is the ability to produce high density plasmas at the lowest possible power consumption. Benefits of high density plasmas include the production of higher grade coatings in general, commonly allow for a greater homogeneity in the coating microstructure,
It has previously been proposed to produce high density plasmas by means of electron cyclotron resonance (ECR) by use of microwaves at a specific frequency of 2.45 GHz and a magnetic field of 875 Gauss in vacuum. Although this produces a useful high density plasma, the size of the required magnetic field is such that it consumes high power levels and is therefore expensive.
A more recent proposal for the production of high density plasmas is the use of high density plasma waves. These can be produced in a chamber at high vacuum, for example 10xe2x88x922 to 10xe2x88x924 mbar, by interaction between a uniform magnetic field and an electric field profile of an external antenna operating at a radio frequency (RF). Wave energy from the antenna emissions is transferred to the electrons produced in a plasma discharge, for example argon, present in the chamber by the well known mechanism known as Landau damping; with these waves, energy exchange is thought to occur in a much more efficient manner than with other types of discharge. This effect can generally be regarded as a collisionless damping of waves in a plasma due to particles in the plasma which possesses a velocity almost equal to the phase velocity of the wave. Such particles tend to travel with the wave without xe2x80x9cseeingxe2x80x9d a rapidly fluctuating electric field and can therefore exchange energy with the wave. A plasma containing electrons some of which are faster and some of which are slower than the wave itself. However, in a Maxwellian distribution, there are more slower electrons than faster ones and therefore there are more particles receiving energy from the plasma wave than those Imparting energy to the wave.
The 13.56 MHz frequency discharge is the most widely used to produce high density plasma waves, although related frequencies of 6.78 MHz and 27.12 MHz could also be employed.
High density plasma wave induced plasma generation can usefully be performed in a chamber in which, or about which, field coils are present to effect a uniform magnetic field (commonly cylindrical) in a pre-determined area of the chamber. Three such field coils in a linear array spaced along the length of the relevant past of the chamber are generally sufficient. At one end of the array is positioned the RF antenna to cause, in use of the process, the intense plasma wave produced by interaction between the magnetic field and the RF power supply, which in turn accelerates plasma electrons by the Landau damping mechanism.
The design of the RF antenna required to produce helicon waves and the subsequent generation of a high density plasma is important. The design has been the subject of considerable discussion in original papers by Boswell and subsequently by Chen and thereafter by various other authors.
Previous designs have centred on complex configurations in which the various operating modes are embodied to provide in particular, a double loop system made from a continuous length of conductive material in which:
a first loop having a main axis which is orientated parallel to the direction of the magnetic field present in the chamber;
a second loop also having a main axis which is orientated parallel to the direction of the magnetic field present in the chamber; and
an electrical cross linkage between the first and second loops of varying design.
Although it has been shown that the use of helicon wave systems can produce high density plasmas to good effect, the complexity of the RF antenna and their spatial configuration with the magnetic array required to Interact with the RF power can provide difficulties in the implementation of helicon wave production of helicon wave plasmas.
The invention is concerned with the provision of high density plasma systems embodying an antenna of simpler, easier to use design and having further benefits as described below.
In accordance with the invention, there is provided a high density plasma forming device having a housing comprising an ionisable gas, a process chamber, a target mounted within the process chamber, at least one source chamber completely open at one end to the process chamber, a radio frequency antenna for the formation of a plasma and a magnetic means by which the plasma may be directed and focused onto the target, wherein the antenna is a helical coil associated with the source chamber such that plasma formed within the source chamber is directed into the process chamber and focused onto the target mounted therein.
In accordance with a further aspect of the invention there is provided a process for forming a high density plasma comprising the steps of:
(i) supplying a helical coil radio frequency antenna with a discharge power supply, and
(ii) inducing the ionisation of an ionisable gas within the source chamber completely open at one end to a process chamber to form a plasma, and
(iii) the direction and focusing of the plasma within a process chamber by magnetic means. The invention also provides a process for forming a high density plasma utilising the apparatus of the invention as described above,
It has surprisingly been found that the use of a helically wound coil antenna, as opposed to the complex antennae used previously, can be used to good effect and to provide additional technical benefits leading to a factor of three enhancement in the plasma ion density. The helically wound coil antenna in the invention generally comprises an electrically conducting strip or wire with one end connected to the RF live supply and the other to earth (ground) and hence returns to an RF matching unit. As such, the RF power passes through each turn of the helically wound coil in turn between the two ends.
The RF supply should generally be within the range of 1 to 200 MHz, however, the preferred frequencies would be within the range 1 to 30 MHz with frequencies of 6.78 MHz, 13.56 MHz and 27.12 MHz the most preferred. In general the RF frequency is surprisingly not crucial as long as the RF source is matched with the antenna in a manner known per se.
The process of the invention should preferably be performed in a chamber made of an insulating material for example a generally cylindrical chamber, wherein quartz and glass are preferred materials.
In preferred embodiments, the helically wound coil antenna can conveniently be wound around the outside of the relevant part of the chamber. This is especially applicable in the case of a metallic, for example brass, strip antenna around an insulating, for example quartz, chamber. The antenna is preferably cooled, for example water cooled.
The helically wound coil antenna preferably comprises at least three turns and advantageously comprises from three to eight turns, for example three, four or five. The RF power applied to the helically wound coil antenna must be matched in a manner known per se to ensure the formation of a high density plasma wave in to the magnetic field present in the chamber.
The antenna must be adapted to generate an RF electric field, interaction between this electric field and the magnetic field generates a modified electric field which in turn produces a plasma wave which couples energy to plasma electrons through a Landau damping mechanism which in turn causes high ionisation of the plasma gas and hence a high intensity plasma.
The magnet means can comprise a single magnet, for example an annular solenoid placed coaxially about the chamber and positioned remote from the antenna to produce, in use a magnetic field relative to the antenna axis between the magnet and the antenna.
Preferably, however, the magnetic means comprises more than one magnet. In preferred embodiments, there is provided a first (source) magnet associated with the antenna and a second (chamber) magnet positioned remote from the antenna to generate a linked magnetic field.
In certain embodiments, the source magnet is preferably an annular solenoid positioned co-axially about the helically wound coil antenna with a small clearance between the external diameter of the antenna and the internal diameter of the solenoid. In some cases, however, the source chamber magnet is slightly removed to the side of the antenna away from the chamber magnet, or even to the opposite side of the process chamber whilst remaining co-axial therewith.
In other preferred embodiments, the process chamber magnet also comprises an annular solenoid positioned co-axially with the antenna axis but having a larger diameter than the source magnet. In order to allow the magnetic lines of flux to bridge the process chamber.
As an example, the source magnet can be such that it generates a magnetic field of the order of 5xc3x9710xe2x88x923 Tesla parallel to the axis of the source chamber and/or antenna and the process chamber magnet can generate a stronger magnetic field of the order of 5xc3x9710xe2x88x922 Tesla again parallel to the central axis of the source chamber and/or antenna with the linked magnetic field being in accordance with the invention.
In the case of the use of solenoid magnet(s) in particular, the current flowing through the solenoid(s) determines the magnetic field strength, the magnetic field gradient across the chamber (if any) and the direction of the overall field. This is critical to the overall operation of the process and the control of its operating parameters.
In general, in the case of using more than one magnet, the source and each chamber magnet must generate respective magnetic fields in the same direction; it should be noted, however, that the process works irrespective of the selected same direction.
An advantage of the invention is the use of a further magnet on one side of the chamber between the source magnet (or the antenna if no source magnet is employed) and the chamber magnet surprisingly allows the plasma beam to be attracted or xe2x80x9csteeredxe2x80x9d in the process chamber towards or away from the further magnet depending on the polarity of the magnet. In such embodiments, the use of a solenoid is advantageous in that switching the solenoid current in one direction will deflect the plasma towards the solenoid and switching the current in the other direction would repel the plasma away from the solenoid.
There may, in certain circumstances, be problems associated with the deposition of material from a target present in the plasma not only on to a substrate held within the vacuum chamber but also on to the internal surfaces of the vacuum chamber, particularly when used for coating processes.
Such problems are generally associated with RF leakage via such coatings on the internal surfaces which has an effect increasingly as the coatings become thicker and/or more widespread of reducing the overall efficiently of the coating process.
In further preferred embodiments of the invention, therefore, it is proposed that the apparatus includes a vacuum process chamber having a source chamber in which the antenna is deployed, with magnet means present in the vicinity of the source chamber and the process chamber to propagate the magnetic field and thereby to generate the high density plasma out of the source chamber and in to the process chamber.
In such an arrangement, a target comprising material to be deposited and a substrate on to which the target material is to deposited can be situated in the process chamber. In such an arrangement, even though some deposition may occur on the process chamber surfaces, relatively low or zero amounts of deposition of target material will generally occur on the interior surfaces of the source chamber where the antenna is positioned. As a result, RF leakage from the RF antenna via chamber surface deposits should be avoided.